what is difference between "Primary key" and "Unique key"?

Question Posted / swapna

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what is difference between "Primary key" and "Unique key"?..

Answer / swapna

1)unique key can be null but primariy key cant be null.
2)primariy key can be refrenced to other table as FK.
Primary key and unique are Entity integrity constraints
Primary key allows each row in a table to be uniquely
identified and ensures that no duplicate rows
exist and no null values are entered.
Unique key constraint is used to prevent the duplication of
key values within the rows of a table and allow null
values. (In oracle, one null is not equal to another null).

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what is difference between "Primary key" and "Unique key"?..

Answer / satya_k21

Hi,

Primary Key: Not Null+ unique.but it wont allows null
values.

Unique: It allows null values , but it should be unique.

Regards,
Satya.k

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what is difference between "Primary key" and "Unique key"?..

Answer / nitin gupta

Uniques: Enforce unique data in the database
it allows row containing null value but only once
It implements non clustered index.

primary key: Enforce unique data in the database
it don't allow any record containing null value
it implements clustered index (data physically
sorted)

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what is difference between "Primary key" and "Unique key"?..

Answer / kk

PRIMARY KEY=UNIQUE KEY+ NOT NULL KEY so its a combo drive

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